In recent years, optical coherence tomography (OCT) has attracted attention as an apparatus for imaging an eye using optical scanning. Unlike an X-ray CT apparatus, optical coherence tomography is noninvasive to human bodies, and is therefore expected to be utilized in the medical field and biological field. For example, in the ophthalmology, apparatuses that form images of a fundus and cornea or the like are in a practical stage.
The apparatus disclosed in Patent Document 1 uses a technique of so-called “Fourier Domain OCT.” That is to say, the apparatus irradiates a low-coherence light beam to an object, superposes the reflected light and the reference light to generate an interference light, and acquires the spectral intensity distribution of the interference light to execute Fourier transform, thereby imaging the morphology in the depth direction (the z-direction) of the object. Furthermore, the apparatus is provided with a galvano mirror that scans a light beam (signal light) along one direction (x-direction) perpendicular to the z-direction, and is thereby configured to form an image of a desired measurement target region of the object. An image formed by this apparatus is a two-dimensional cross sectional image in the depth direction (z-direction) along the scanning direction (x-direction) of the light beam. The technique of this type is also called Spectral Domain.
Patent Document 2 discloses a technique of scanning with a signal light in the horizontal direction (x-direction) and the vertical direction (y-direction) to form multiple two-dimensional cross sectional images in the horizontal direction, and acquiring and imaging three-dimensional cross sectional information of a measured range based on the cross sectional images. As the three-dimensional imaging, for example, a method of arranging and displaying multiple cross sectional images in the vertical direction (referred to as stack data or the like), or a method of executing a rendering process on volume data (voxel data) based on stack data to form a three-dimensional image may be considered.
Patent Documents 3 and 4 disclose other types of OCT apparatuses. Patent Document 3 describes an OCT apparatus that images the morphology of an object by sweeping the wavelength of light that is irradiated to an object (wavelength sweeping), detecting interference light obtained by superposing the reflected lights of the light of the respective wavelengths on the reference light to acquire its spectral intensity distribution, and executing Fourier transform. Such an OCT apparatus is called a Swept Source type or the like. The Swept Source type is a kind of the Fourier Domain type.
Further, Patent Document 4 describes an OCT device that irradiates a light having a predetermined beam diameter to an object and analyzes the components of an interference light obtained by superposing the reflected light and the reference light, thereby forming an image of the object in a cross-section orthogonal to the travelling direction of the light. Such an OCT device is called a full-field type, en-face type or the like.
Patent Document 5 discloses an example of applying OCT to the ophthalmologic field. It should be noted that, before OCT was applied, a retinal camera, a slit lamp microscope, a scanning laser opthalmoscope (SLO) etc. were used as apparatuses for observing an eye (see Patent Documents 6, 7 and 8 for example). The retinal camera is an apparatus that photographs the fundus by projecting illumination light onto the eye and receiving the reflected light from the fundus. The slit lamp microscope is an apparatus that obtains an image of the cross-section of the cornea by cutting off the light section of the cornea using slit light. The SLO is an apparatus that images morphology of retinal surface by scanning a fundus by laser light and detecting reflected light thereof with a highly sensitive imaging element such as a photomultiplier.
The apparatus with OCT is superior relative to the retinal camera, etc. in that high-definition images can be obtained, further in that cross sectional images and three-dimensional images can be obtained, etc.
Thus, the apparatus using OCT can be used for observation of various regions of the eye and is capable of obtaining high-definition images, and therefore, has been applied to the diagnosis of various ophthalmologic disorders.
Now, in various medical fields, examinations of same sites of a subject are repeated (sometimes referred to as follow up). Examples of follow up include progress observations and preoperative/postoperative observations. There are specific problems in ophthalmology regarding follow up. Specifically, it is difficult to examine a same site repeatedly because of influences of eye movement (involuntary eye movement during fixation, rotation, etc.) of a subject eye. In particular, reproducing the same scanning position as a past examination is extremely difficult for an imaging apparatus of the type that scans an eye with light.
Patent Documents 9 to 11 discloses technologies that can be applied for solving such a problem of follow up imaging.
The purpose of an invention disclosed in Patent Document 9 is to smoothly detect misregistration (including rotational transfer) between fundus images. For this purpose, this invention is configured to detect an amount of misregistration between fundus images by collating a plurality of subregion images cut from the respective fundus images.
The purpose of an invention disclosed in Patent Document 10 is to preferably detect retinal endogenous signals of a human eye by continuously measuring the same cross sectional site of a fundus. For this purpose, this invention is configured to detect displacements of scanning positions by irradiating and detecting a second light beam while scanning the fundus with a measuring light beam and control a second optical scanner based on the detection results, thereby correcting the scanning positions of the measuring light beam as required. Further, it is also described that template matching based on characteristic regions in retinal front images is utilized for detecting misregistration of scanning positions.
The purpose of an invention disclosed in Patent Document 11 is to correctly irradiate a light beam to a preset position on a fundus regardless of eye movement. For this purpose, this invention is configured to detect picture angle information of scanning light and misregistration information of the fundus based on fundus front images and correct scanning positions of scanning light based on these information.